Title of Invention

PROCESS FOR THE INTEGRATED PREPARATION OF AMMONIA AND UREA

Abstract

A process for the combined preparation of urea and ammonia reactant by steps of providing ammonia synthesis gas containing carbon dioxide and conversion of the synthesis gas to the ammonia reactant, reacting the ammonia reactant with the carbon dioxide in the synthesis gas to ammonium carbamate and to urea product, which process comprises further steps of prior to the conversion of the synthesis gas to the ammonia reactant, (i) washing the synthesis gas with an aqueous solution of the ammonia reactant and forming a solution being rich in the ammonium carbamate; (ii) removing excess of the ammonia reactant from the washed synthesis gas by washing with water and withdrawing an aqueous solution of the ammonia reactant; (iii) purifying the water washed synthesis gas by removing remaining amounts of water and ammonia; and (iv) passing the purified synthesis gas to the conversion of the synthesis gas to the ammonia

Full Text

The present invention is directed towards production of urea by integrating ammonia reactant preparation in the urea synthesis process. At present urea is made from ammonium carbamate according to the reactions: 2 NH3 +CO2 — NH2CO2NH4 NH2CO2NH4 H2O +H2NCONH2 Most urea is manufactured in connection with synthetic ammonia plants since the necessary carbon dioxide is available from the synthesis gas purification system in front end of the ammonia plant. A large part of the world"s ammonia production is utilised for the manufacture of urea. Thus, an integrated process for production of urea will have technical and economical advantages. However, with the present technology the production of urea and ammonia takes place in largely independent plants. US Patent No. 3,349,126 discloses a process for production of ammonia and urea, where the CO2 containing synthesis gas stream is split up in two streams, one stream where isolated CO2 is sent to a urea reactor, and another stream, where the CO2 in the synthesis gas is converted to ammonium carbamate in an aqueous solution of ammonium carbamate. Washed synthesis gas from a second CO2 wash tower under goes a methanisation. Ammonia formed in the ammonia synthesis section is cooled, condensed, separated from the gas phase and pumped to the urea reactor. US Patent No. 3,303, 215 discloses a process for preparation of ammonia and urea, where ammonium carbamate and urea is formed in the same tower and solutions with urea are recycled to the ammonium carbamate / urea reactor. The formed ammonia is condensed and separated from the gas phase. Liquid ammonia is then pumped to the CO2 wash tower. US Patent No. 4,320,103 discloses a process for production of ammonia and urea, where the synthesis gas is washed in two CO2 wash towers, with concentrated ammonia water in the first one and with an ammonium carbamate rich in ammonia in the second one. A methanator is installed to remove remaining CO and CO2 as meth ane, and ammonia is recycled from the urea section to the ammonia loop at the inlet side of the ammonia reactor. Thereby remaining CO and CO2 are converted to methane, which is an inert in the downstream syntheses of ammonia, ammonium carbamate and urea and thereby a loss. Ammonia in ammonia reactor feed results in a considerable lower conversion of synthesis gas to ammonia or in higher requirement to catalyst volume or to recirculator capacity, as the conversion is an equilibrium reaction. The general object of this invention is to provide an improved process for the preparation of urea by the above reaction of carbon dioxide with ammonia to ammonium carbamate and carbamate decomposition to urea product, wherein ammo nia production and urea production is combined in a single flow scheme by integrating carbon dioxide and ammonia reactant preparation into synthesis gas purification of the ammonia synthesis. In accordance with the general object, the invention provides a process for the combined preparation of urea and ammonia reactant by steps of providing ammonia synthesis gas containing carbon dioxide and conversion of the synthesis gas to the ammonia reactant, reacting the ammonia reactant with the carbon dioxide in the synthesis gas to ammonium carbamate and to urea product, which process comprises further steps of: prior to the conversion of the synthesis gas to the ammonia reactant, (i) washing the synthesis gas with an agueous solution of the ammonia reactant and forming a solution being rich in ammonium carbamate; (ii) removing excess of ammonia reactant from the washed synthesis gas by washing with water and withdrawing an aqueous solution of ammonia reactant; (iii) purifying the water washed synthesis gas by adsorbing remaining amounts of water and ammonia; and (iv) passing the purified synthesis gas to the conversion of the gas to ammonia reactant. The attached block diagram describes an integrated process for the preparation of urea from carbon dioxide and ammonia reactant according to a specific embodiment of the invention. A gas preparation unit consisting of a conventional HDS, Reforming and Shift Section is followed by a unit where the gas with a high CO2 content is brought up to a pressure suitable for ammonia synthesis. The major part of CO2 is then removed by washing the gas with an aqueous ammonia so- lution. The carbamate solution formed flows to the urea reactor. The dissolved gases in the solution can from this point be recycled to the suction side of the compressor. The main flow of stripped gas contains small amounts of components such as CO, NH3 and O2, and is now utilised as make-up gas to the ammonia synthesis. First, a CO selective oxidation removes remaining CO by selective conversion: CO + "/2O2 -^ CO2. The effluent gas from the oxidation is saturated with NH3 and is mixed with the effluent gas from the ammonia converter. The combined stream is lead to a wash, where the formed amounts of CO2 in the gas are converted with NH3 to dissolved ammonium carbamate, and the main part of the ammonia is removed by washing with water. Since this process is exothermal, a two stage process is preferred: In the first stage the gas is in contact with a relative strong ammonia solution, which absorbs about half of the amount of ammonia. In the second stage, the final absorption takes place with water originating from the urea unit. By the above selective oxidation of CO, this catalyst poison is converted to CO2, which ends as product urea. The gas leaving the absorption unit contains some ammonia, but also some water, which must be removed upstream the ammonia converter. This process may be carried out in a mass absorber by molecular sieves or alumina. The mass absorber can be regenerated by purge gas. Downstream the mass absorber, the gas passes the recirculation compressor and is then passed to a conventional synthesis unit consisting of hot heat exchanger, synthesis converter and steam generation. After cooling to ambient temperature, the gas is passed to water absorption as described above. Since the carbamate reaction takes place at elevated temperatures and since a large amount of water is formed during the process, it is not required to produce low temperature, anhydrous ammonia for this purpose. Evidently, a too high water content in the produced ammonia will have a negative influence on the urea process, because the water will reduce the yield of urea and thus increase the recycle. Thus, a certain upgrading of the ammonia concentration is required. In the conventional ammonia process there is usually a surplus of low temperature calories which are utilised in the ammonia concentration unit operating at a moderate pressure, typically at 15-17 kg/cm^ g. At this pressure a significant increase in ammonia concentration up to 85% will be obtained by distillation at low temperature, below 120°C. The process eliminates the need for two compressors (refrigeration and CO2) , a refrigeration circuit and a CO2 removal in the ammonia plant. Since the NH3 concentration at converter inlet will be close to nil, a reduction in converter size or a decreased recycle rate is obtained. Large part of the equipment in the urea plant is to recover CO2 and NH3. CO2 is removed by scrubbing with ammonia; water scrubbing and distillation subsequently remove the ammonia. Finally, "inerts" mainly hydrogen and nitrogen are discarded to atmosphere. By recycling the inert gase5 to the suction side of the syngas compressor, a simplification of the process and a reduction of the number of units in the urea plant could be achieved. Further, the urea plant employs a process condensate stripping system that purifies the process water to make it suitable for boiler feed water. In the ammonia plant a similar system is employed and an integration of the two systems may have some potential benefits. WE CLAIM; 1. A process for the combined preparation of urea and ammonia reactant by steps of providing ammonia synthesis gas containing carbon dioxide and conversion of the synthesis gas to the ammonia reactant, reacting the ammonia reactant with the carbon dioxide in the synthesis gas to ammonium carbamate and conversion to urea product characterised in that the process comprises steps of prior to the conversion of the synthesis gas to the ammonia reactant, (i) washing the synthesis gas with an aqueous solution of the ammonia reactant so that the carbon dioxide in the synthesis gas reacts with the ammonia reactant and forming a solution being rich in the ammonium carbamate; (ii) removing excess of the ammonia reactant from the washed synthesis gas by washing with water and withdrawing an aqueous solution of the ammonia reactant; (iii) purifying the water washed synthesis gas by removing remaining amounts of water and ammonia; and (iv) passing the purified synthesis gas to the conversion of the synthesis gas to the ammonia reactant. 2. The process as claimed in claim 1, wherein an effluent stream from the conversion of the synthesis gas to the ammonia reactant is combined with the washed synthesis gas prior to the washing with water. 3. The process as claimed in claim 1 or 2, wherein CO in the washed synthesis gas is selectively oxidised.

Documents:

761-mas-2001 abstract-duplicate.pdf

761-mas-2001 abstract.pdf

761-mas-2001 claims-duplicate.pdf

761-mas-2001 claims.pdf

761-mas-2001 correspondence-others.pdf

761-mas-2001 correspondence-po.pdf

761-mas-2001 description(complete)-duplicate.pdf

761-mas-2001 description(complete).pdf

761-mas-2001 drawings.pdf

761-mas-2001 form-1.pdf

761-mas-2001 form-18.pdf

761-mas-2001 form-26.pdf

761-mas-2001 form-3.pdf

761-mas-2001 form-5.pdf

761-mas-2001 others.pdf

761-mas-2001 petition.pdf